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Stand dynamics during drought: responses of adult trees, tree regeneration, and understory vegetation to multiyear drought in pinyon-juniper woodlands
AuthorFlake, Samuel W.
AdvisorWeisberg, Peter J.
Natural Resources and Environmental Science
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Widespread dieoff of trees associated with severe drought is a recent global phenomenon of increasing conservation and management concern. Tree dieoff is likely to produce dramatic and widespread alterations to plant community composition and successional dynamics, with associated changes in nutrient cycling, hydrology, and wildlife habitat. In order to predict and manage the effects of future widespread drought, there is a need for greater understanding of both the causes of drought-associated tree mortality as well as the subsequent effects of mortality on tree regeneration and understory dynamics. In this study, we investigated the effects of climate, stand structure, and insect herbivores on tree mortality and canopy dieback of semi-arid pinyon-juniper woodlands, as well as the implications of woodland overstory mortality on the abundance of juvenile trees and the distribution of understory plants. In 2015 we resampled 102 plots in a permanent plot network established in 2005 spanning pinyon-juniper woodlands in eleven mountain ranges in central Nevada, encompassing strong gradients of topography, elevation, soil conditions, and aridity. Using individually-tagged trees, we measured tree survival and changes to canopy greenness. In order to assess the effects of neighborhood tree density on changes in tree canopy health, we recorded high-precision GPS locations for each tree and calculated the basal area of neighboring trees and the distance to the nearest neighbor. We measured tree regeneration in two size classes: seedlings (<20 cm tall) and sapling (>20cm tall). We also measured understory cover by species in 1-m2 quadrats. We noted the microhabitat of tree juveniles and quadrats in order to assess the importance of fine-scale microhabitat heterogeneity to understory dynamics.This study is the first to document widespread dieoff and canopy decline in Great Basin pinyon-juniper woodlands in response to recent severe drought. We found mortality was most strongly predicted by high growing-season vapor pressure deficit and low winter precipitation (Forest Drought Stress Index), while canopy decline was most closely associated with cumulative climatic water deficit. Trees in sites with deeper soils also experienced greater health declines. Stand structure had a smaller, but significant, effect on tree survival and canopy health, with trees in dense 4-m neighborhoods and those with close neighbors experiencing increased mortality risk and canopy dieback.As predicted, the regeneration of pinyon pine was strongly influenced by changes to the overstory trees. Both seedlings and saplings were positively associated with plot-level dead tree basal area and with fine-scale mortality-associated microhabitats, suggesting that drought mortality may create canopy gaps with increased recruitment. In contrast to microhabitat results, pinyon seedlings were much more abundant in stands with lower levels of canopy dieback and in stands with more live basal area, suggesting that new recruitment may be limited by reduced seed availability. The dynamics of tree regeneration in woodlands will likely depend upon seed availability, resource levels, timescale, and the abundance and spatial arrangement of larger juveniles (advance regeneration) present before the drought.We found little evidence of response to tree mortality for most understory plant functional types. However, Poa secunda and Bromus tectorum were both more abundant than expected in mortality-associated microhabitats. At a plot scale, Bromus tectorum abundance was positively associated with canopy dieback, and has increased in dominance since 2005 in arid sites with high levels of dieback. While some native species may respond positively to resources released by tree mortality, there is a risk of invasion and increasing dominance of cheatgrass under future droughts, underlining the importance of understanding overstory-understory interactions in arid woodlands.This study highlights the need for further research into the effects of within-stand structural heterogeneity on woodland responses to drought. Pinyon-juniper woodlands have a complex, patchy distribution of trees which likely affects the ways in which competition may predispose trees to drought mortality. Overstory mortality creates heterogeneous microhabitats which alter the distribution of juvenile trees as well as understory species, thus spatially structuring the response of these species to drought. Aridity and severe drought are likely to increase in coming decades, and understanding the complex interactions that drive woodland tree mortality and the subsequent understory response is critical to the effective management of resilient woodlands.